|Use sfc /scannow as first option for any system files that go astray.|
Click Start menu and in the search bar type:
Hit Enter key
On the entry for cmd.exe that appears, right click it and select "Run as Administrator". Next type:
Note the one space before /scannow - Hit Enter key
That will replace any missing system files including the main rundll32.exe in "c:\windows\system32". If it was another one elsewhere and belonged to a particular program then you might have to re-install whatever program stops working.
Be VERY careful about ever downloading system files. If they are not scams they are likely to be the wrong version. They can vary not only with operating system but your state of updates.
Always pop back and let us know the outcome - thanks